1. Field of the Invention
The present invention relates to an oscillator obtained by arranging a surface acoustic wave (SAW) element and an IC element having an oscillation circuit in the same package.
2. Description of the Related Art
In a conventional oscillator using a SAW element, the SAW element and an oscillation circuit are arranged in different packages. Therefore, the SAW element and the oscillation circuit are electrically connected to each other through an external pattern such as a printed board.
With recent advances in semiconductor techniques, however, integration of high-frequency oscillation circuits can be performed. Japanese Patent Application No. sho 63-325348 discloses a circuit suitable for integration of oscillators using SAW elements.
Japanese Patent Application No. hei 1-60162 discloses a modulator incorporating a plurality of oscillators.
With such a technical background, in order to realize a more compact oscillator with higher reliability, an oscillator, obtained by arranging a SAW element and an oscillation circuit in the same package has been studied.
In an oscillator obtained by arranging a SAW element and an oscillation circuit in the same package, various problems are posed as follows.
Wiring for connecting a SAW element chip to an oscillation circuit chip can be mostly easily performed by using bond wires. If, however, bond wires are not properly arranged, the bond wires become long. As a result, the parasitic inductance may be increased, or the oscillator becomes susceptible to external electromagnetic induction. For this reason, an operation of the oscillation circuit becomes unstable, and operation errors may be caused. In addition, if the bond wires are long, their weights are increased. Therefore, if vibrations are exerted on the oscillator, the bond wires may be disconnected or detached from the circuit.
A differential amplifier is suitable for an oscillation circuit used in an integrated oscillator. FIG. 1 shows an oscillation circuit having a differential amplifier. Transistors 502 and 504 constitute a transistor pair for performing differential amplification. The collectors of the transistors 502 and 504 are respectively connected to one end of a DC power source 510 through resistors 506 and 508 having the same resistance. The emitters of the transistors 502 and 504 are connected to the other end of the DC power source 510 through a common DC current source 512. The bases of the transistors 502 and 504 are respectively connected to the output terminals of a bias circuit 518 through resistors 514 and 516 having the same resistance. The bias circuit 518 receives power from the DC power source 510. In the differential amplifier having the above-described arrangement, one port 522 of a two-port type SAW resonator 520 is arranged between the collectors, as the output terminals of the differential amplifier, of the transistors 502 and 504. The other port 524 of the SAW resonator 520 is connected between the bases, as the input terminals of the differential amplifier, of the transistors 502 and 504. Output terminals 526 and 528 are respectively connected to the collectors of the transistors 502 and 504.
In the amplifier having the above-described arrangement, a signal is positively fed back from the output terminals to the input terminals of the differential amplifier through the two-port type SAW resonator 520, thus performing oscillation. Oscillation outputs are extracted, as differential outputs, from the output terminals 526 and 528.
If an oscillation circuit constitutes a differential amplifier, no RF current flows in a DC power source 510 in principle, as disclosed in Japanese Patent Application No. sho 63-3253. In addition, since such a circuit requires no capacitor, it is suitable for an IC. However, an RF current does not flow in the DC power source 510 only when the circuit is operated in a good condition. For example, if the positive and negative input or output impedances of the differential amplifier are not in balance, an RF current flows in the DC power source 510. This RF current acts as noise to other circuits connected to the DC power source 510, and hence is not desirable.
In the differential amplifier, noise from a power source line or external electromagnetic induction can be canceled by a differential operation. Two pairs of positive and negative input and output lines, i.e., a total of four lines extend outward from the differential amplifier. For this reason, positive and negative wires for differential input and output may differ in length from each other. If the lengths of the positive and negative wires differ from each other, positive and negative impedances may differ in magnitude from each other, or an oscillating operation may become unstable due to noise from the power source line or external electromagnetic induction.
In an oscillator obtained by arranging a plurality of oscillation circuits in the same chip, the oscillation circuits are preferably arranged as close to each other as possible in order to obtain uniform electrical characteristics and temperature characteristics. However, if the plurality of oscillation circuits are arranged to be too close to each other, mutual interference tends to occur between the oscillation circuits through the bond wires. Especially, if the plurality of oscillation circuits have different oscillation frequencies, a given oscillation circuit may be oscillated at the oscillation frequency of another oscillation circuit due to a pull-in phenomenon, or a spurious output may be generated in an oscillation spectrum due to cross modulation.
Such an oscillator has just begun to be studied. Hence, sufficient studies of the shape and position of each chip and of a wiring method of chips for an optimal design have not yet made.